1. Field of the Invention
The present invention relates to electrochemical cells generating electrical energy by means of a chemical reaction. Electrolytic cells, for example of the lithium/silver vanadium oxide (Li/SVO) type, are typically constructed of one or more layers of anode, separator, and cathode. A screen or foil current collector is enclosed in the anode and cathode to transport electrons. An electrode assembly may be built by stacking multiple layers or plates on top of each other or by winding one or more long strips of the stacked layers around a mandrel. The electrode assembly is placed inside a case and immersed in an electrolyte, which transports ions.
The number of electrode layers in a cell is a trade-off between current and capacity requirements. More plates or winds of the electrodes give more surface area between the anode and cathode and subsequently, higher current capability to the cell. However, less plates or winds require less passive parts (separator and screen or foil) and allow for more active material (cathode and anode), which results in higher capacity.
Safety is another important consideration in selecting design options involving plates, winds and the nature of the electrical connections. Particularly important is a design option that enhances transport of heat out of the cell. This is critical for batteries used to power implantable medical devices such as cardiac pacemakers and defibrillators. During an internal electrical short, electrical energy is converted to heat energy. To facilitate heat dissipation, it is desirable to conduct heat outside the cell as rapidly as possible so that internal temperature does not exceed the melting point of lithium. Such a temperature rise could cause hazardous venting of the cell.
2. Prior Art
FIG. 1 shows a traditionally wound Li/SVO cell 10 having a cathode connection 12 to the insulated terminal pin (not shown) and anode connections 14 to the cell casing (not shown). One end of the unitary wound anode is in the center and the other end is connected to the case wall. If there were an internal short at the center, bottom of the electrode assembly, one pathway would be for the heat to travel the full length and height of the anode to leave the cell.
FIG. 2 shows another form of a traditionally wound cell 20, often referred to as a galaxy-wound cell, having cathode connection tabs 22 and anode connection tabs 24. One of each of the two anode ends is in the center and one of each of the other anode ends is connected to the case wall. If there were an internal short in such a galaxy-wound cell, a similar heat travel pathway from the center, bottom of the electrode assembly is approximately half that of the traditionally wound cell.
FIG. 3 shows a traditional Li/SVO cell 30 having the cathode 32 made up of individual plates connected in parallel and the anode 34 wrapped around the cathode plates in a serpentine manner. Tabs 36 to a bridge-like connection structure 38 connect the cathode plates together and tabs 40 connect the anode to the cell casing (not shown). Heat transfer out of this traditional-plate cell design is similar to that of the galaxy-wound cell shown in FIG. 2.
However, there is a need to develop an electrode arrangement that enhances heat transfer from the cell in the event of an internal short circuit. This extends the applicability of current electrochemical cells to new varieties of applications.